I personally tried this and im close to unlocking him.
This guide is ONLY for those not willing to wait for Gaara to appear on sign up rewards or pay for him BUT you WONT make it to 4 star
First: Always check the black market for fragments. They are usually 100000 coins = 5 fragtments
Always check the suvival, group and e.t.c shops they sometimes have him in 5 fragmentts
That is all you need to do BUT they will add gaara at sign up in one month just as they did the chinese version so if youre lazy you could wait even if it is 6 months.